Make promise tracing opt-in for better performance

RSVP used to send a stack trace with every promise by default.   Now is is disabled by default due to the high performance cost.

This PR adds a checkbox called "Trace promises" to enable the RSVP flag `instrument-with-promises` to start storing stack traces along with promise info. 

https://github.com/tildeio/rsvp.js/pull/303

cc @stefanpenner 
This value only changes through the checkbox.  Could the value change more than once in a run loop?